The term "image recording" is conventionally taken to mean a process which produces a spatial pattern of optical absorption in the recording medium. Photographic processes are well known examples of this type of process.
In a broader sense, however, the word "image" means a spatial variation of the optical properties of a sample in such a way as to cause a desired modification of a beam of light passing through, or reflecting from, the sample. Refractive index images in general and holograms in particular, which modulate the phase, rather than the amplitude, of the beam passing through them are usually referred to as phase holograms. Phase holographic image recording systems produce a spatial pattern of varying refractive index rather than optical absorption in the recording medium and, thus, modulate a beam of light without absorbing it. This type of refractive index image formation includes a number of optical elements or devices, such as holographic lenses, gratings, mirrors, and optical waveguides, which superficially bear little resemblance to absorption images.
Holography is a form of optical information storage. The general principles are described in a number of references, e.g., "Photography by Laser" by E. N. Leith and J. Upatnieks in Scientific American, 212, No. 6,24-35 (June, 1965). A useful discussion of holography is presented in "Holography", by C. C. Guest, in Encyclopedia of Physical Science and Technology, Vol. 6, pp. 507-519, R. A. Meyers, Ed., Academic Press, Orlando, Fla., 1987. In brief, the object to be photographed or imaged is illuminated with coherent light (e.g., from a laser) and a light sensitive recording medium (e.g., a photographic plate) is positioned so as to receive light reflected from the object. This beam of reflected light is known as the object beam. At the same time, a portion of the coherent light is directed to the recording medium, bypassing the object. This beam is known as the reference beam. The interference pattern that results from the interaction of the reference beam and the object beam impinging on the recording medium is recorded in the recording medium. When the processed recording medium is subsequently appropriately illuminated and observed at the appropriate angle, the light from the illuminating source is diffracted by the interference pattern recorded in the recording medium to reconstruct the wavefront that originally reached the recording medium from the object. Thus, the hologram resembles a window through which the virtual image of the object is observed in full three-dimensional form, complete with parallax.
Holograms that are formed by allowing the reference and object beams to enter the recording medium from the same side are known as transmission holograms. Interaction of the object and reference beams in the recording medium forms fringes of material with varying refractive indices which are approximately normal to the plane of the recording medium. When the hologram is played back by viewing with transmitted light, these fringes refract the light to produce the viewed virtual image. Such transmission holograms may be produced by methods which are well known in the art, such as disclosed in Leith and Upatnieks, U.S. Pat. Nos. 3,506,327; 3,838,903 and 3,894,787. A diffraction grating is the simplest possible transmission hologram. It is the hologram of two coherent plane waves. It can be created by splitting a single laser beam and recombining the beams at the recording medium.
Holograms formed by allowing the reference and object beams to enter the recording medium from opposite sides, so that they are traveling in approximately opposite directions, are known as reflection holograms. Interaction of the object and reference beams in the recording medium forms fringes of material with varying refractive indices which are, approximately, in planes parallel to the plane of the recording medium. When the hologram is played back these fringes act as partial mirrors reflecting incident light back to the viewer. Hence, the hologram is viewed in reflection rather than in transmission. Since the wavelength selectivity of this type of hologram is very high, white light may be used for reconstruction.
Reflection holograms may be produced by an on-axis method wherein the beam of coherent radiation is projected through the recording medium onto an object therebehind. In this instance, the reflected object beam returns and intersects with the projected beam in the plane of the recording medium to form fringes substantially parallel to the plane of the medium. Reflection holograms also may be produced by an off-axis method wherein a reference beam is projected on one side of the recording medium and an object beam is projected on the reverse side of the medium. In this instance the object beam is formed by illuminating the object with coherent radiation which has not passed through the recording medium. Rather, the original beam of coherent radiation is split into two portions, one portion being projected on the medium and the other portion being directed to project on the object behind the medium. Reflection holograms produced by an off-axis process are disclosed in U.S. Pat. No. 3,532,406.
A holographic mirror is the simplest possible reflection hologram. It is the hologram of two coherent plane waves intersecting in a recording medium from substantially opposite directions. It can be created by splitting a single laser beam and recombining the beams at the recording medium, or the unsplit laser beam can be projected through the medium onto a plane mirror therebehind. A set of uniformly spaced fringes are formed that have a sinusoidal-like intensity distribution. The fringes are oriented parallel to the bisector of the obtuse angle between the two beams propagating in the recording medium. If the obtuse angle is 180.degree. and the beams are normal to the plane of the medium, the fringes will be parallel to the plane of the medium. If the two beams do not make equal angles with the normal to the plane of the medium, then the fringes which are formed will be slanted at an acute angle relative to the plane of the medium. The holographic mirror can be characterized by its wavelength of maximum reflection and by its reflection efficiency, that is, by the percent of incident radiation which is reflected at its wavelength of maximum reflection.
The substantially horizontal fringes which form reflection holograms are much more difficult to record than the perpendicular fringes which form transmission holograms for two reasons. The first reason is the need for higher resolution, i.e., the need for more fringes per unit length, and thus a closer fringe spacing. Horizontal reflection holograms require about 3X to 6X more fringes per unit length than do transmission holograms. The second reason is the sensitivity of horizontal fringes to shrinkage of the recording medium during exposure. Any shrinkage of the recording medium during exposure will tend to wash out the fringes and, if severe, will prevent a hologram from being formed. This is in contrast to the transmission hologram case, where shrinkage has little or no effect when the fringes are perpendicular to the plane of the medium, and produces only relatively minor image distortion if the transmission fringes are slanted more than 45.degree. from the plane of the medium.
A variety of materials have been used to record holograms. Among the more important are: silver halide emulsions, hardened dichromated gelatin, ferroelectric crystals, photopolymers, photochromics and photodichroics. Characteristics of these materials are given in Volume Holography and Volume Gratings, by L. Solymar and D. J. Cook, Chapter 10, Academic Press, New York, 1981, pp. 254-304.
Dichromated gelatin is the material most widely used for recording holograms. This material has become the popular choice because of its high diffraction efficiency and low noise characteristics. However, dichromated gelatin has poor shelf life and requires wet processing. Plates must be freshly prepared, or prehardened gelating must be used. Wet processing means that an additional step is required in hologram preparation and may also cause the hologram to change due to swelling and then shrinkage of the gelating during processing. Shrinkage may particularly be a problem when preparing reflection holograms in that the shrinkage will alter the wavelength of maximum reflection. The requirement that plates by freshly prepared each time a hologram is made plus the problems associated with wet processing make reproducibility extremely difficult to achieve with dichromated gelatin.
While early holograms where prepared from silver halide or dichromated colloids which required several processing steps, photopolymerizable elements have been proposed which require only a single process step. U.S. Pat. No. 3,658,526 to Haugh discloses preparation of stable, high-resolution holograms from solid, photopolymerizable layers by a single step-process wherein a permanent refractive index image is obtained by a single imagewise exposure of the photopolymerizable layer to actinic radiation bearing holographic information. The holographic image formed is not destroyed by subsequent uniform actinic exposure, but rather is fixed or enhanced.
Despite the many advantages of the solid photopolymerizable layers proposed by Haugh, reflection holograms produced therefrom have been poor at best, with little reflection efficiency. Thus, there is a need for improved compositions for the preparation of reflection holograms.